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    <script>
        //看代码,说出输出的结果
        /**
         *  回顾:
         *      boolean的false总共有四个
         *          number  =>  0
         *          null
         *          undefined
         *          ""      ''
         *      字符串的单引号与双引号
         *          "今天天气好晴朗,'处处'好风光,好风光"
         */
        var username = null;
        if(username){//原因的是if语句的括号里面要求是boolean类型,如果是boolean,那么一切好说,如果不是boolean,那么隐式类型的转换
            console.log("我是法外狂徒张三");
        }else{
            console.log("我不是张三");
        }
    </script>
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